Check if revision exists before downloading

The `sync` command is fairly slow even if the cache contains every revision being
synced. This is because a download is attempted even if it is unneeded.

This change changes the flow of `context.Sync` to attempt to `RevisionSync` with
whatever is in the cache, if that fails then it calls `Download` followed by
another `RevisionSync`.

context/sync.go

@@ -356,17 +356,22 @@ func (ctx *Context) Sync(dryrun bool) (err error) {
 				continue
 			}
 		} else {
+			// Use cache.
 			vcsCmd = updateVcsCmd(sysVcsCmd)
-			err = vcsCmd.Download(repoRootDir)
-			if err != nil {
-				rem = append(rem, remoteFailure{Msg: "failed to download repo", Path: vp.Path, Err: err})
-				continue
-			}
 
 			err = vcsCmd.RevisionSync(repoRootDir, vp.Revision)
+			// If revision was not found in the cache, download and try again.
 			if err != nil {
-				rem = append(rem, remoteFailure{Msg: "failed to sync repo to " + vp.Revision, Path: vp.Path, Err: err})
-				continue
+				err = vcsCmd.Download(repoRootDir)
+				if err != nil {
+					rem = append(rem, remoteFailure{Msg: "failed to download repo", Path: vp.Path, Err: err})
+					continue
+				}
+				err = vcsCmd.RevisionSync(repoRootDir, vp.Revision)
+				if err != nil {
+					rem = append(rem, remoteFailure{Msg: "failed to sync repo to " + vp.Revision, Path: vp.Path, Err: err})
+					continue
+				}
 			}
 		}
 		dest := filepath.Join(ctx.RootDir, ctx.VendorFolder, pathos.SlashToFilepath(vp.Path))